1.Field of the Invention
The invention relates to a device for detecting partial discharges in winding elements of an electrical machine, in which device the winding element to be investigated is sampled over its length, with high voltage applied, by means of a drivable probe and the radio frequency signals initiated by the partial discharges are received by means of an antenna and are detected and evaluated in a test set connected downstream.
The point of origin for the invention is a prior art as results, for example, from the CIGRE Report "Diagnosis of HV Machine Insulation -- From Inspection to Continuous Monitoring" by A. Kelen, CIGRE Symposium 05-87, Vienna 1987, pages 1020-05.
2. Discussion of Background
The electrical insulation systems of the stator windings in rotating electrical machines are subject to aging processes which are mainly caused by mechanical, thermal and electrical stresses. These stresses produce weak points in the associated insulating materials, in which dangerous partial discharges (PD) occur in the event of an operating stress. The so-called slot discharges are the most worrying PD phenomena in the machine insulation system. Early detection and localization of the slot discharges protects the power generators and consumers against uncontrolled failures in power supply networks on the one hand and prevents destruction of the insulation on the other hand, since most defects which are discovered early are repairable.
In the CIGRE report quoted initially, the author provides a comprehensive summary of the measurement processes and devices which were known at the time for detecting partial discharges. However, the report is limited to a rather summary representation of the processes and equipment used hitherto. A more detailed report on partial discharges and their diagnosis, and the interpretation of the measurement results, is published in CIGRE Paper 15/33-12, B. Fruth, J. Fuhr "Partial Discharge Pattern Recognition -- A Tool for Diagnosis and Monitoring of Aging", distributed at the CIGRE 1990 Session, 26 August - 1 September 1990.
Partial discharges occur only in the event of sufficient electrical stress (rated voltage) of the insulation. In order to be able to identify a slot discharge unambiguously, it is necessary to work with high voltage applied to the stator winding. However, this is linked to endangering the test personnel and, as a rule, requires the rotor to be removed.